Concentrated milk products are often desired because they allow for smaller quantities of milk to be stored and transported, thereby resulting in decreased storage and shipping costs. Concentrated milk products are generally prepared from whole milk, partly skimmed milk, skim milk, and combinations thereof.
Membrane filtration technology (e.g., microfiltration, ultrafiltration, and the like) is employed in the dairy industry to provide selective separation, concentration, and purification of protein components from a liquid dairy product such as raw milk. Membrane filtration technology involves separating the liquid dairy product into a first liquid component (i.e., permeate or ultrafiltrate) and a second liquid component (i.e., concentrate or retentate) by placing the dairy product in contact with a semi-permeable membrane. The liquid dairy product is usually contacted with the semi-permeable membrane under pressure and is typically subjected to turbulent flow so as to agitate the liquid adjacent the membrane and thereby allowing water and small molecular weight solutes (e.g., lactose) to pass through the membrane in order to obtain higher solids in the retained liquid concentrate. The concentrate, which does not flow through the semi-permeable membrane, has a higher protein concentration than the permeate. In conventional ultrafiltration processes, the permeate is collected and typically disposed of on the farm, used as livestock feed, disposed to the city wastewater, or converted to other forms. The retentate is typically treated until a milk concentrate with the desired enriched concentration factor and/or solids content is obtained.
On-farm ultrafiltration generally allows reduced volumes for the resulting concentrated milk products, thereby lowering shipping, storage, and refrigeration costs. Thus, milk intended for shipment is often concentrated by a factor of about 3 times (based on total solids) to obtain these benefits. Modern membrane filtration technology, however, allows efficient production of more than about 5 times (based on concentration of total solids) milk concentrates, which would, of course, allow for additional cost savings since less water would be shipped. However, 3.5 times and higher concentrates (especially 5 times and higher) are likely to gel during transportation or storage. Such age gelation characteristics generally limit the use of such higher concentrated products unless special and costly handling systems are available. For example, the concentrates can be heated at low temperatures prior to pumping but at the risk of increasing the microbial content of the concentrate so these products are typically used in cheese-making processes right after pumping.
Therefore, currently available concentrated milk products that are transported over long distances are generally limited to those having a total solids content below about 30 percent and a shelf life of about 5 days.